In the late 1980's, the research of the late Professor John Walsh at Dartmouth College and others led to the development of tunable, electron beam driven radiation sources to produce electromagnetic radiation at terahertz frequencies in a flexible, tunable, and affordable fashion. Walsh's tunable terahertz lasers are fully described in U.S. Pat. Nos. 5,263,043 and 5,790,585 (hereinafter referred to as “the Walsh patents”). Walsh's work showed that a small, compact, and relatively inexpensive table top free electron laser which utilizes a diffraction grating element could be a commercially practical device to generate far infrared (FIR) or terahertz electromagnetic waves.
The Walsh patents essentially disclose FEL apparatuses that pass an electron beam over a surface of a diffraction grating element (i.e., a resonator) thereby producing electromagnetic radiation. The diffraction grating elements of the Walsh patents are fabricated by machining slots in an aluminum block. With this manufacturing approach, the minimum width of the slots largely depends upon the width of the saw blade used. This can limit the slot widths that can be fabricated. Additionally, the choice of block materials may also be limited since some materials cannot be cleanly machined without chipping or burring occurring at the edges (e.g., in the case of hard materials). Others may tend to flow when machined (e.g., in the case of softer materials). Availability of grating slot dimensions may also be limited as saw blades cannot be made to be arbitrarily thin. Furthermore, the depth and the shape of the interior profile of the slots and the shape of the external surfaces between the slots are not easily controlled in these manufacturing techniques. The saw blade, in its cutting action, may leave burrs on exterior comers and rounded surfaces on interior comers. According to the theory developed by Walsh, the width, depth and shape of the slots is critical in determining the wavelength of operation and the strength of the coupling between the bound electromagnetic radiation modes and the electron beam.